Multiple radio signal paths result from signal reflections from objects such as buildings or vehicles so that a receiver may receive more than one copy of a transmitted signal. The different copies are often delayed in time due to different signal path propagation lengths. Such an environment is shown in FIG. 1. By using devices such as RAKE receivers, different copies of the signal from one transmitter that are separated in time can be received and coherently combined to produce a coherent received signal comprising the majority of the received signal power. However, if there are multiple, non time aligned signals these do result in mutual interference. To some extent, these multipath environments can be compensated for, particularly in relatively static networks such as fixed wireless access networks, where the receivers and transmitters are stationary. In fixed wireless access (FWA) networks, the multipath environment changes only relatively slowly, for example due to the growth of leaves on trees, and these changes can be accommodated to some extent by the arrangements described below. In a FWA environment, the rapid changes in the multipath environment, such as those due to movement of vehicles, are minimised by placing subscriber terminal antennas at (or above) roof top level.
The applicant's co-pending U.S. patent application Ser. No. 09/078,010 (Baines) published as EP-A-0957593 discloses a technique for time alignment of uplink CDMA signals for a static (FWA) or low mobility communication system. Baines discloses the concept of a base station signalling to subscriber terminals to alter their transmission timings for the uplink signals back to the base station such that the multiple received copies of a signal from a subscriber terminal are translated in time. By controlling the time translation of these signal copies, or components, from a number of subscriber terminals such that the main (strongest) signal component from each subscriber terminal is time aligned with the main signal component of the other subscriber stations, the effect of cross-interference between the terminals can be minimised. This is of particular advantage when the CDMA codes used by the individual subscriber terminals are designed to be orthogonal—that the integral of the product of the two codes over one (or more) full intervals of the code is zero. A well known example of orthogonal CDMA codes are the Walsh-Hadamard functions, commonly used in the downlink of CDMA mobile systems (i.e. TIA standard IS-95), (see, for example, chapter 8 of “Principles of Mobile Communication” by Gordon L. Stuber, Kluwer Academic Publishers 1995). As shown in FIG. 2, by time translating the transmissions on channel B, the main multipath signal component received on channel B can be aligned with that received on channel A from another subscriber terminal thereby eliminating the most significant interference effects between these two subscriber terminals.
The concepts disclosed in Baines may be implemented in local wireless loop or fixed wireless access (FWA) systems as synchronous code division multiple access (SCDMA). The SCDMA concepts may also be applied for terminals in a low mobility mobile environment such as an indoor mall. The 3GPP international wireless standards body for 3rd Generation Mobile Systems, for example, has been discussing a SCDMA concept for mobile terminals in which the base station (also known as “Node-B”) sends a time alignment (tracking) command every 200 milliseconds instructing a subscriber terminal to advance or delay its transmission by ⅛ of a chip. This process is described in the 3GPP standards document 3GPP TSG RAN WG 1 #19 document TR25.854 version 0.2.0 (Study Report on USTS) dated 27 Feb.–2 Mar. 2001. This document discloses an uplink synchronous transmission scheme (USTS) for low mobility terminals, especially intended for indoor and dense pedestrian environments. USTS reduces uplink intracell interference by orthogonalising the signals received from various subscriber terminals by instructing these subscriber terminals to adjust their transmission times such that the strongest received components from each terminal are synchronised. The scheme utilises a time alignment bit sent to a subscriber terminal by the base station. This bit instructs the subscriber terminal to advance or delay its transmission by ⅛ of a chip. For example a terminal moving away from the base station will increase its signal propagation time and may require its transmission timing to be advanced to compensate. By sending a series of these bits, the base station may instruct the mobile terminal to adjust its transmission timing to the desired alignment and to track changes in the alignment that may be necessary over time.
The system works by initially determining the round trip propagation delay between the base station and a subscriber terminal, the base station then setting an initial synchronisation parameter (TINIT_SYNC) to compensate for the delay. Once a call is in progress, the synchronisation of the transmission timing can be adjusted to take account of changes in the multipath environment or the terminal's location that may alter the received time of the multiple copies of a signal from the subscriber terminal by using a time alignment bit (TAB). A proposal for standardizing this in the 3GPP system is to replace the terminal power control bit (PCB) with a TAB every two frames (20 milliseconds). As there may be errors during the transmission of the TABs, at the subscriber terminal, a number of TABs are combined over a 200 millisecond interval. If the average exceeds a threshold value the transmission is delayed by a predetermined amount (⅛ chip), whereas if the average is less than or equal to a threshold the transmission time is advanced by a predetermined amount. Other combinations of TAB and averaging intervals may alternatively be used depending on the rate of change of the multipath conditions or the motion of the terminal.
Whilst this and similar arrangements have been shown to work reasonably well in a slow changing multipath environment, they are inadequate for high mobility systems where received copies or components of a signal can change rapidly due to movement of the mobile subscriber terminals. As a mobile subscriber terminal moves about the environment, the main or highest power received signal component may suddenly fade and a previously low power signal component may become the dominant one. Furthermore, some signal components may disappear altogether and new ones may appear within a short span of time. This is due to the mobile moving amongst reflecting objects such that a reflected path may cease to exist as the mobile terminal moves into the “shadow” of a building for example. Similarly, because the mobile subscriber terminals are typically located near the ground, a reflection may cease to exist when a mobile object such as a vehicle moves past the mobile terminal. A tracking process as discussed for the standard only adjusts the transmission timing of a subscriber unit by ⅛ chip every 200 milliseconds. It would take some time to change transmission timing to align a new component of the signal and during this time interference persists. As the new component may be offset in time by up to a few microseconds, sending time alignment commands every 200 milliseconds with a step size of ⅛ chip is too slow in a mobile environment. (In the 3GPP standard ⅛ of a chip has a duration of approximately 32 nanoseconds.)